Gate valves positively controlled from the outside generally comprise a rod or pin with a free end proximal to the gate and an opposite end secured to the head of a double-acting piston in a cylinder to which air or some other fluid under pressure is alternately admitted from opposite sides. Since the heat generated in the hot-runner structure travels through the valve rod to the piston head, overheating of the piston-and-cylinder assembly must be prevented by maintaining good thermal contact between the cylinder and a massive heat sink such as a cover plate exposed to the ambient atmosphere. To minimize the wasteful dissipation of heat by the cover plate, it is desirable to separate same from the hot-runner structure by an air gap-maintained by spacers of low thermal conductivity. For the sake of compactness, however, the width of that air gap must be limited.
Advantageously, the fluid cylinder itself can be used as such a spacer if its periheral wall is relatively thin to provide the necessary thermal resistance. In that case, however, a considerable heat differential develops along the cylinder axis. Thus, pressure fluid alternately admitted into the cylinder from its cold end and from its hot end may give rise to thermal stresses and may also have a detrimental effect upon the usual packing ring or rings forming a seal between the piston head and the inner cylinder wall.